Fix a Second Monitor or HDMI Display Not Detected on Linux

# list every display connector and its state
for p in /sys/class/drm/card*-*; do echo "$p: $(cat $p/status)"; done
# which session type are you in?
echo $XDG_SESSION_TYPE

Identify your case

"Not detected" covers several different problems. Match what you see to a cause before changing anything:

Diagnose step by step

Step 1: Check what the kernel sees

The kernel's DRM subsystem lists every physical connector. This is independent of GNOME, KDE or X11, so it tells you where the problem is.

for p in /sys/class/drm/card*-*; do echo "$p: $(cat $p/status)"; done

Example output:

/sys/class/drm/card1-DP-1: disconnected
/sys/class/drm/card1-eDP-1: connected
/sys/class/drm/card1-HDMI-A-1: disconnected
/sys/class/drm/card2-HDMI-A-2: connected

• Your monitor's port is connected: the hardware and driver are fine. Go to Step 3 and enable it in the desktop.
• It stays disconnected: check cable, monitor input, then the GPU driver (Step 2).
• The port is missing entirely: the GPU it hangs off has no driver loaded. On hybrid laptops this is the NVIDIA case.

Two cards (card1, card2) mean two GPUs. Note which card the external port belongs to. Kernel messages often show why a link failed:

sudo dmesg | grep -iE 'drm|hdmi|edid|link training' | tail -30

Lines like link training failed or EDID block 0 is all zeroes point at the cable, adapter or dock rather than at software.

Step 2: Check which GPU drives the port

lspci -k | grep -A3 -E 'VGA|3D|Display'

Every GPU should show a Kernel driver in use: line (i915 or xe for Intel, amdgpu for AMD, nvidia or nouveau for NVIDIA). A GPU without that line has no driver, and any port wired to it is dead.

Rule out the simple things before going deeper: try another cable (HDMI 2.0/2.1 and DisplayPort 1.4 cables differ), select the correct input on the monitor, and try the other port. Cheap USB-C to HDMI adapters and KVM switches are a frequent cause of missing EDID data.

Fix: NVIDIA hybrid graphics (Optimus laptops)

On most gaming and workstation laptops the internal screen is driven by the Intel or AMD iGPU while the HDMI and often a USB-C port are wired directly to the NVIDIA GPU. With nouveau or no driver, those ports never come up.

nvidia-smi
cat /sys/module/nvidia_drm/parameters/modeset

• If nvidia-smi fails, the proprietary or open NVIDIA module is not loaded. Install it properly: on Ubuntu follow installing NVIDIA drivers on Ubuntu; if you got a black screen afterwards, use the NVIDIA black screen fix. A load failure caused by Secure Boot is covered in Secure Boot and MOK enrollment.
• modeset must print Y for external outputs to work under Wayland. Recent driver releases enable it by default; if yours prints N, add nvidia-drm.modeset=1 as a kernel parameter.

Check the PRIME mode on Ubuntu and Mint. In intel (power saving) mode the NVIDIA GPU is switched off and its ports cannot work:

prime-select query
sudo prime-select on-demand
sudo reboot

on-demand keeps the iGPU for the desktop and still lets the NVIDIA GPU drive its own connectors. Some laptops also have a BIOS setting (often called "Hybrid", "Discrete" or "MUX switch"); setting it to Discrete is a reliable workaround if hybrid mode keeps failing.

Fix: the monitor is detected but not shown

If /sys/class/drm says connected, the problem is only configuration. How you fix it depends on the session type from the quick fix.

GNOME (Wayland)

Open Settings → Displays. Choose Join rather than Mirror, and check that the external display is not switched off. Press Super+P to cycle display modes. GNOME 50 has no X11 session, so xrandr cannot change anything here. If the layout is corrupt, move the saved configuration away and log in again:

mv ~/.config/monitors.xml ~/.config/monitors.xml.bak

KDE Plasma

Use System Settings → Display & Monitor, or the command-line tool that works on both Wayland and X11:

kscreen-doctor -o
kscreen-doctor output.HDMI-A-1.enable output.HDMI-A-1.position.1920,0

X11 sessions (Xfce, Cinnamon, MATE, Plasma on X11)

xrandr
xrandr --output HDMI-1 --auto --right-of eDP-1

Use the output names xrandr prints; they can differ from the kernel names (HDMI-1 vs HDMI-A-1). If the right resolution is missing because EDID is not read, add a mode manually:

cvt 2560 1440 60
# copy the name and numbers from the "Modeline" line cvt prints
xrandr --newmode "2560x1440_60.00" 312.25 2560 2752 3024 3488 1440 1443 1448 1493 -hsync +vsync
xrandr --addmode HDMI-1 "2560x1440_60.00"
xrandr --output HDMI-1 --mode "2560x1440_60.00"

Sway and other wlroots compositors

wlr-randr
wlr-randr --output HDMI-A-1 --on --mode 1920x1080 --pos 1920,0

Make it permanent with an output block in your Sway config.

Fix: wrong resolution, flicker or "input not supported"

If the monitor is detected but runs at a low resolution, flickers, or shows "input not supported", the kernel probably could not read its EDID (the block of data in which the monitor describes its supported modes). Check what the kernel received:

sudo apt install edid-decode    # dnf / pacman / zypper: edid-decode
edid-decode /sys/class/drm/card1-HDMI-A-1/edid

An empty file or a checksum error means the EDID is not getting through. Typical reasons are passive adapters (especially DisplayPort to HDMI), KVM switches, HDMI splitters and very long cables. Connect the monitor directly with a known-good cable to confirm.

A high refresh rate can also exceed what the cable or port can carry. A 4K monitor at 120 or 144 Hz needs HDMI 2.1 or DisplayPort 1.4 on both ends; with an older cable it may fall back to 30 Hz or lose signal. Select a lower refresh rate in display settings to test. For flicker on hybrid laptops, try the monitor on a port wired to the iGPU (often a USB-C port) to see whether the problem follows the GPU.

Fix: USB-C docks, Thunderbolt and DisplayLink

Docks fail in three distinct ways:

1. Thunderbolt not authorized. Run boltctl list. If the dock shows status: connected but not authorized, authorize it with boltctl enroll <uuid> or in Settings → Privacy.
2. DisplayLink dock. Check lsusb | grep -i displaylink. DisplayLink devices do not use DP Alt Mode; they need Synaptics' proprietary evdi-based driver. Without it the dock's network and USB ports work, but its video outputs never appear.
3. No DP Alt Mode. Not every USB-C port carries video. Check the laptop manual or the symbol next to the port (DisplayPort or Thunderbolt logo).

Daisy-chained DisplayPort (MST) hubs work with Intel and AMD; with NVIDIA, connect directly when testing.

Fix: regression after an update

If the monitor worked last week, boot the previous kernel from Advanced options in the GRUB menu. If it works there, the new kernel or a driver that failed to rebuild is at fault:

uname -r
dkms status

Stay on the older kernel until an update lands, and report the regression to your distro. If the kernel detects the port but refuses to light it, you can force the connector on with a parameter such as video=HDMI-A-1:e (use the name from /sys/class/drm, without the cardN- prefix).

Distro notes

Last resort

• Create a new user and log in. If the monitor works there, delete ~/.config/monitors.xml (GNOME) or ~/.local/share/kscreen (KDE) for your normal user.
• Try the other session type if your desktop offers one (Plasma still has X11). The differences are explained in Wayland vs X11.
• Boot a current live USB of Fedora or Ubuntu. If the monitor works there, your installed system has a driver or config problem; if it does not, suspect the cable, adapter or the port itself.
• Update the laptop firmware with fwupdmgr refresh && fwupdmgr update; Thunderbolt and USB-C controller firmware fixes are common.

Once both screens work, if you see tearing on one of them, see fixing screen tearing.

Frequently asked questions